Spatial and seasonal variability of CH4 in the eastern Gulf of Cadiz (SW Iberian Peninsula)

Abstract Methane (CH 4 ) concentrations were measured along three sections of the eastern Gulf of Cadiz (designated “Guadalquivir”, “Sancti Petri” and “Trafalgar”) during eight cruises in 2014 and 2015. The concentration of CH 4 varied from 3.6 to 19.7 nmol kg − 1 (CH 4 saturation percent of 122 and 916%), showing seasonal variation. The highest values were found in December 2014 and November 2015. In most of the sampling area the highest concentration of CH 4 was found in subsurface waters at depths close to the thermocline, and in the bottom waters near the coast. The seawater-air flux of CH 4 ranged between 0.8 and 59.7 μmol m − 2  d − 1 , showing seasonal variation in function of the temperature of the surface water. In the “Guadalquivir” and “Sancti Petri” sections, the CH 4 fluxes increased with proximity to the coast; this may be a result of continental inputs and CH 4 emissions from sediments. The whole study area behaves as a source of CH 4 to the atmosphere with mean values of 0.5 and 0.6 Gg CH 4  yr − 1 in 2014 and 2015, respectively.